A rock bolt is a crucial tool for construction companies working with rock, stone and other dense, natural materials. Most commonly used in tunnel building and reinforcement, these long bolts work to increase a structure's stability. With a unique physical appearance and three specific anchoring types, a rock bolt is one of the most commonly used anchoring techniques around the world.
The origin of the rock bolt is unknown, but it is believed to have originated with mining companies during the late 1800s. The first documented use of rock bolts was at the Saint Joseph Lead Mine in the United States in the 1940s. Since that time, these bolts have gone on to worldwide acclaim and their use has developed into sophisticated techniques for tunnel building and more.
A rock bolt comes in many styles and lengths, but they generally are at least 13 feet (4 m) in length and about an inch (25 mm) in circumference. The bolts transfer pressure from a rock structure's unstable face to its more stable core in order to prevent cave-ins. The bolts usually are constructed of steel, because of the metal's ability to accept large amounts of stress and pressure. In addition, rock bolts are used together in various patterns to better secure rock and prevent it from collapsing. Different patterns have been developed for different rock formations and different needs.
Before a pattern can be created, the hole must first be forged. A rock bolt is not inserted directly into raw rock because it would damage the bolt's structure. Instead, a rock bolt anchor has a hole drilled for it before insertion. Rock bolts are used for anchoring the surrounding rock, and the process uses one of three major types of anchors, either mechanical, grouted or friction.
Mechanical rock anchors are the most common type of rock bolt. These long rods actually expand within the hole when they are twisted in order to activate a mechanism. This mechanism increases its size, assuring a snug fit that can be counted on to remove pressure from the surface of the rock.
A grouted rock bolt uses a standard piece of steel as the anchoring mechanism but does not have any mechanical ability to spread itself out in the hole. Instead, the rod is inserted and then surrounded by a cement-like bonding grout. After the grout hardens, the bolt is fused to the rock and relieves its stress.
Friction rock bolts are a newer technology in rock anchoring. The bolt produces a radial force against the bore hole and prevents the rod's slippage. This allows for less mess and less time than grout and has less chance of mechanical error than mechanical bolts.
By Patrick Wensink